It is known that reaction of olefins with carbon monoxide and hydrogen (hydroformylation) can be used to prepare aldehydes and alcohols which contain one carbon atom more than the starting olefin. The reaction is catalyzed by hydrido-metal carbonyls, preferably of the metals of groups 8, 9, and 10 of the Periodic Table (corresponding to the IUPAC recommendation of 1985). Apart from cobalt, which was originally used as the catalyst metal and has been widely used in industry, rhodium has recently been gaining increasing importance. In contrast to cobalt, rhodium allows the reaction to be carried out at low pressure. In addition, terminal olefins preferentially form n-aldehydes and only lesser amounts of isoaldehydes. Finally, the hydrogenation of the olefins to give saturated hydrocarbons is also significantly lower in the presence of rhodium catalysts than with cobalt catalysts. In the processes introduced in industry, the rhodium catalyst is used in the form of modified hydrido-rhodium carbonyls which contain additional ligands, in particular tertiary organic phosphines or phosphites. The cobalt catalysts are also used in the form of carbonyls which additionally contain phosphines or phosphites as ligands, even though this variant of the hydroformylation is of lesser industrial importance than processes in which rhodium serves as catalyst.
The ligands which control the activity of the catalyst metal (also referred to as control ligands) are usually present in excess of the amount required for formation of the complex, and thereby stabilize the complex by the law of mass action. The catalyst system therefore comprises a complex and free a ligand, and the ligand is important, not only for the specific catalytic activity, but also for the stability of the complex.
The hydroformylation reaction can be carried out either in homogeneous or in heterogeneous systems. In the homogeneously catalyzed reaction, the catalyst is dissolved in the reaction product and a solvent may also be present. This procedure has proven itself well both when using cobalt catalysts and when using rhodium catalysts. However, difficulties are presented in the separation of the reaction products and, in the case of the reaction catalyzed by rhodium, in the recovery of the catalyst. Product and catalyst solutions are customarily separated from one another by distillation. However, owing to the thermal sensitivity of the aldehydes and alcohols formed, this route can only be used in the hydroformylation of lower olefins, i.e. olefins having up to about 8 carbon atoms in the molecule.
The indicated deficiencies are avoided in the rhodium-catalyzed reaction by using water-soluble rhodium complexes as catalysts. Such a process is described, for example, in DE-C 26 27 354. The solubility of the rhodium complexes is here achieved by use of sulfonated triarylphosphines as constituents thereof. In this embodiment, separation of the catalyst from the reaction product after the reaction is complete is carried out simply by separation of aqueous and organic phases, i.e., without distillation, and thus without additional thermal process steps. A further feature of this procedure is that n-aldehydes are formed with particularly high selectivity from terminal olefins, and isoaldehydes are formed only in very much smaller amounts. The complexing constituents used for water-soluble rhodium complexes are preferably sulfonated triarylphosphines and additionally carboxylated triarylphosphines.
Organic phosphines, regardless of whether they are soluble in organic media or in water, have proven themselves well in industrial practice as control ligands owing to their wide variety, their catalytic activity, and their selectivity. Nevertheless, a series of disadvantages stands in the way of their wider use. These include, in particular, the oxidation sensitivity which occurs especially in the presence of metals and metal ions. Therefore, when using catalysts based on phosphine-containing complexes, measures have to be taken to exclude oxidizing agents such as oxygen or air so as to reduce the losses of the ligands, which can frequently only be prepared at high cost. A further property which all organic phosphines have in common, which limits their possible uses, is the irreversible cleavage of phosphorus-carbon bonds; for example, in hydroformylation this occurs to an increased extent above certain temperatures depending on the type of phosphine. It leads to deactivation of the catalyst and thus to high phosphine consumption which impairs the economics of the method. Finally, the conventional alkylphosphines and arylphosphines, like the organic phosphites similarly used as ligands, do not allow coverage of the entire range of the electronic control possibilities in respect of the catalytically active metal centers. In particular, there is a lack of strongly nucleophilic electron-rich ligands which are resistant to oxidizing agents and form stable bonds to the metal.